The wiring method of the thermal resistance

Thermal resistance is a kind of temperature detector commonly used in medium and low temperature regions. Thermal resistance thermometry is based on the characteristic that the resistance value of metal conductors increases with temperature. Its main features are high measurement accuracy and stable performance. Among them, platinum thermal resistance has the highest measurement accuracy. It is not only widely used in industrial temperature measurement, but also made into a standard benchmark. Thermal resistors are mostly made of pure metal materials, and platinum and copper are the most widely used. In addition, nickel, manganese and rhodium have begun to be used to manufacture thermal resistors. There are many types of temperature-sensing materials commonly used in metal thermal resistors, and platinum wire is commonly used. In addition to platinum wire, metal thermal resistance materials for industrial measurement include copper, nickel, iron, iron-nickel, etc.

The thermal resistance is a primary element that converts the temperature change into the resistance value change. Usually, it is necessary to transmit the resistance signal to the computer control device or other primary instrument through the lead wire. The industrial thermal resistance is installed on the production site, and there is a certain distance between it and the control room, so the lead wire of the thermal resistance will have a greater impact on the measurement results.

There are three main ways to wire the thermal resistance:

Two-wire system: The method of connecting a wire at each end of the thermal resistance to draw out the resistance signal is called the two-wire system: this lead method is very simple, but because the connecting wire must have a lead resistance r, the size of r is different from the material and length of the wire. related factors, so this lead method is only suitable for occasions with low measurement accuracy

Three-wire system: The method of connecting one lead at one end of the root of the thermal resistance and connecting two leads at the other end is called a three-wire system. This method is usually used in conjunction with a bridge, which can better eliminate the influence of lead resistance. It is an industrial commonly used in process control.

Four-wire system: The method of connecting two wires at each end of the root of the thermal resistance is called a four-wire system, in which two leads provide a constant current I for the thermal resistance, convert R into a voltage signal U, and then pass the other two leads. Lead U to the secondary meter. It can be seen that this lead method can completely eliminate the influence of lead resistance, and is mainly used for high-precision temperature detection.

The thermal resistance adopts the three-wire connection method. The three-wire system is used to eliminate the measurement error caused by the resistance of the connecting wires. This is because the circuit for measuring the RTD is generally an unbalanced bridge. The thermal resistance is used as a bridge arm resistance of the bridge, and its connecting wire (from the thermal resistance to the central control room) also becomes a part of the bridge arm resistance. This part of the resistance is unknown and changes with the ambient temperature, resulting in measurement errors. Using a three-wire system, one wire is connected to the power end of the bridge, and the other two are connected to the bridge arm where the thermal resistance is located and the bridge arm adjacent to it, which eliminates the measurement error caused by the wire line resistance.